Conventional techniques exist for encoding digital data in the active video areas of video signals having either the NTSC format (525 lines per frame, with field rate equal to 60 Hz) or the PAL format (625 lines per frame, with field rate equal to 50 Hz). In one class of such conventional techniques, audio signals are digitized and encoded in still frame audio ("SFA") format, or still frame audio tape ("SFAT") format, in the active video areas of a video signal. Alternatively, digital data other than digitized audio signals may occupy the active video areas of an SFA or SFAT signal.
The SFA format will be described with reference to FIGS. 1 through 4. The SFAT format will be described with reference to FIGS. 1, 5, and 6.
FIG. 1 represents a block of SFA or SFAT data. The data block of FIG. 1 occupies not more than 39 frames of a standard NTSC or PAL video signal. A black burst signal occupies the first four frames of the data block. The next n frames (where n is an integer greater than or equal to two and less than or equal to thirty-two) comprise digitized encoded audio data. Each such frame includes 7.2 kilobytes of data. If a still picture is to be displayed during playback of the audio signal, the final three frames of the data block comprise a conventional still picture video signal. A header code is recorded at the beginning of the first audio data frame (frame 5 in FIG. 1), and a trailer code is recorded at the end of the last audio data frame. In order to play back a series of recorded data blocks of the type shown in FIG. 1, an appropriately programmed computer system is typically employed to ensure that the audio data is reproduced in the proper sequence and is matched with the proper images.
The format of a single frame of SFA encoded audio data will be described with reference to FIGS. 2 and 3. Although the frame shown in FIGS. 2 and 3 is an NTSC implementation of a frame of SFA encoded data, the PAL implementation is very similar, and the differences between the PAL and NTSC implementations will be apparent from the following discussion. The audio data in an SFA frame occupies two fields. As shown in FIG. 2, in the NTSC implementation each field comprises 240 lines. Each line contains 15 bytes of SFA encoded audio data, so that a total of 3600 bytes of encoded audio data occupy each field. The audio data occupying each field has been digitized (typically with 12 bit resolution, at a sampling rate of 8 kHz) and then subjected to an adaptive differential pulse code modulation process (typically with 4 bit resolution, at a sampling rate of 8 kHz). FIG. 3 shows the first 20 lines of the FIG. 2 frame, and the lines numbered 262 through 284 between the two fields of the FIG. 2 frame. Line 21 is reserved for the header, and line 525 is reserved for the trailer. A white flag signal occupies line 11, forty bits of code occupy each of lines 10 and 273, and twenty-four bits of code occupy each of lines 17 and 18.
FIG. 4 represents a single line comprising one of the fields of an NTSC implementation of a frame of SFA encoded data. The first (left-most) 10.725 microsecond interval of the line includes the horizontal synchronization signal shown in FIG. 4. Digitized, binary encoded audio data (or other binary encoded digital data) occupy the next 50.84 microsecond interval of the line. The final 1.97 microsecond interval consists of a signal having substantially zero IRE amplitude.
FIG. 5 shows a line of SFAT data. The first (left-most) 11.92 microseconds represent the horizontal blanking interval. Two 8-bit synchronization bytes occupy the next 16 bits (approximately the next 2 microseconds) of the line. The first of these synchronization bytes is known as "F0H" and has the form shown in FIG. 5(a). The second of these synchronization bytes is known as "E2H" and has the form shown in FIG. 5(a). The next 184 bits of the line (from bit 112 to bit 296) comprise duobinary encoded audio data. The next byte (from bit 296 to bit 304) is another "E2H" synchronization byte. The next 184 bits (from bit 304 to bit 488) comprise duobinary encoded audio data. The final 24 bits of the line comprise a horizontal blanking signal.
A block of SFAT data has the overall arrangement shown in FIG. 1. Unlike a block of SFA data, however, five header lines are provided at the start of the first frame of still frame audio data in a block of SFAT data. In contrast, in an SFA data block, only one header line is provided at the start of the first frame of still frame audio data.
FIG. 5(b) is an example of one of the five header lines which occupy the first frame of encoded audio data in a block of SFAT data. The first (left-most) 11.92 microseconds of the header line represent the horizontal blanking interval, as in the data line shown in FIG. 5. Two eight-bit synchronization bytes occupy the next sixteen bits (the next 2 microseconds) of the header line. The first of these synchronization bytes is an "F0H" byte and has the form shown in FIG. 5(a). The second of these synchronization bytes is an "E2H" byte and has the form shown in FIG. 5(a). The next 40 bits of the header line (from bit 112 to bit 152) comprise a five-byte identification code. Following the next 64 bits, there are two eight-bit cyclic redundancy check (CRC) codes and another eight-bit "E2H" synchronization code. After this "E2H" code, the next 40 bits (bits 240 through 280) comprise a second five-byte identification code. Then, after the next 64 bits, there are two eight-bit CRC codes, followed by an eight-bit "E2H" code, in turn followed by a third five-byte identification code (occupying bits 368 through 408). Finally, after the next 64 bits, there is a final pair of eight-bit CRC codes, followed by 2.98 microseconds of horizontal blanking signal as in the data line shown in FIG. 5.
The format of a single frame of SFAT encoded audio data will be described with reference to FIG. 6. Each line shown in FIG. 6 is identified by two line numbers, one (in the right column of line numbers) corresponding to an NTSC implementation of the SFAT format, and the other (in the left column of line numbers) corresponding to a PAL implementation of the SFA format. The audio data in an SFAT frame occupies two fields. In the NTSC implementation, the first field comprises 238 lines the second field comprises 242 lines. In the PAL implementation, each field comprises 240 lines. The five lines immediately preceding the first field of data (lines 20 through 24 in the NTSC implementation, and lines 26 through 30 in the
implementation), are reserved for headers (each having format as shown in FIG. 5(b). In both the PAL and NTSC implementations, the data occupying each field is grouped into three data blocks. For example, in the PAL implementation, the data occupying lines 31 through 110 (or lines 339 through 418) comprises a first block; the data occupying lines 111 through 190 (or lines 419 through 498) comprises a second block; and the data occupying lines 191 through 270 (or lines 499 through 578) comprises a third block.
Since the data occupying the active video areas of an SFAT frame is duobinary encoded, the data may be encoded twice, using two conventional error correction codes ECC1 and ECC2. This is in contrast with data occupying the active video areas of a frame of SFA data, which may practically be encoded only once, using a conventional error correction code ECC1.
In the process of video disk manufacturing, it is conventional to generate a master video tape on which SFA encoded audio signals (or other SFA encoded digital data) are recorded. An error correction code (ECC1) employed to encode the data in the active video areas of the recorded signal facilitates assessment of the quality of the master video tape, in a manner to be described with reference to FIG. 7.
In the conventional video disk manufacturing system of FIG. 7, an SFA format video signal having digitized audio data or other digital data in its active video areas is encoded using error correction code ECC1 in encoding unit 1, and the encoded signal emerging from unit 1 is recorded on video tape in SFA format in mastering tape unit 2. Within quality check unit 3, the master video tape produced in unit 2 (which has SFA format) is played in a video tape player equipped with a conventional SFA decoding unit (such as a DB-2040 video disk decoding board, available from Sony Corporation). The decoding unit employs the error correction code ECC1 in a conventional manner to determine the error rate for each block of SFA-encoded data recorded on the master tape. If the error rates are sufficiently low, the master tape is passed to disk replication unit 4. In unit 4, the information on the master tape is transferred to a video disk in SFA format. This information, in SFA format, may be recovered from the disk by a conventional video disk player equipped with a conventional SFA decoding unit 5 (which may be of the same type as is included in quality check unit 3).
Conventional SFA decoding units 5 are capable of employing the ECC1 code of an SFA-encoded signal to correct errors introduced during the combined tape mastering, quality assessment, and disk replication processes performed in units 2, 3, and 4.
A disadvantage of the conventional video disk mastering technique described with reference to FIG. 7 is that the quality assessment operation inherently adds wear and tear to the master tape during playback in unit 3, and so itself contributes to error creation though it is intended to quantify the errors introduced during production of the master video tape.